Conductive adhesive rework method

ABSTRACT

A method of removing cured conductive polymer adhesives, disclosed here as thermal interface materials, from electronic components for reclamation or recovery of usable parts of module assemblies, particularly high cost semiconductor devices, heat sinks and other module components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/709,518, filed on May 11, 2004, now U.S. Pat. No. 7,312,261. Thesubject matter of which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

This invention relates to conductive adhesives for the thermal interfacebetween a silicon device and a heat sink/heat spreader inmicroelectronic assemblies. More particularly, this invention isdirected to conductive adhesives with improved functional performanceand a method to rework the cured adhesives to allow recovery, recycle,or reuse of the heat sink assembly components without causing anydetriment to the device chip or the chip carrier.

The rapid technology advancements in high performance electronicspackaging has focused on reduced size and higher operating speed. Thishas resulted in excessive heat generation during device operation. Thereis an accompanying need for effective heat dissipation methods tomaintain the reliable functional performance of electronic assemblyproducts. The commonly used methods of cooling include helium filledmodules, solder thermal interfaces, thermal greases, elastomericsilicone gels, thermoplastic polymers with thermally conductive fillerssuch as AIN, BN, ZnO, and more recently, phase change materials (PCM),and conductive adhesives. These provide the thermal interface betweenthe silicon device chip and a high thermal conductivity metal heatspreader or heat sink to allow a path for heat dissipation from the highpower density circuit devices during operation.

Thermal grease is spread as a thin layer between the back of the die andthe heat sink. Thermal grease has low thermal resistance and can beeasily reworked. However, it is subject to pump-down and drying whichcauses voids at the interface. This degrades the device performance withtime due to an increase in interfacial resistance. The phase changematerials (PCM) are low melting waxes. Examples include paraffin wax,having graphite particles dispersed in the wax polymer matrix, andsilicone based waxes, such as alkyl methyl silicones, which can be usedas preformed tapes or melt dispensed across interfaces. They provide lowthermal impedance and high thermal conductivity, typically in the range5 W/m K in thin bond line thickness. However the pre-cut films of thesematerials are fragile and also have the problem of performancedegradation and variability, delamination, bleed-out, out-gasing, andgenerally require fasteners, clips or screws to hold the PCM in place.

Another category of thermal interface materials are conductive adhesiveswhich can be used as a thin adhesive interlayer between the heat sink orthe heat spreader and the back side of a silicon die in a flip-chipmodule assembly. The commercially available conductive adhesives aretypically Ag-filled and ceramic-filled epoxy based materials includingflexible epoxies. They are medium to high modulus adhesives (>100,000psi at room temperature). It is generally known that cured coatings ofsuch materials have high intrinsic stress which can cause disruption ofinterface integrity due to delamination. This results in increasedcontact resistance with a corresponding decrease in the heat dissipationeffectiveness at the interface. The commercially available Ag-filledadhesives also have no simple and practical rework method available.Therefore they cannot be readily removed or reworked from contactingsurfaces. The non-reworkability of these adhesives present a seriousdrawback in that it does not allow for defect repair or componentrecovery, recycle or reuse of high cost semiconductor devices, heatsinks and substrates.

The most desired improvements in the thermal interface materialproperties include: ability to form thin bond line with uniformthickness across interfaces, low thermal impedance, low stress andcompliant systems for interface integrity during device operation,stable interfacial contact resistance in T/H (temperature-humidity) andT/C (thermal cycling), TCR stability (temperature coefficient ofresistance), and reworkability for defect repair and reclamation of highcost module components. The preferred materials should also be amenableto removal from contacting surfaces to allow rework without causing anydetriment to the module materials for defect repair, chip replacement,and recovery of high cost components, particularly special type heatspreaders having high thermal conductivity and chip joined modules forreclamation and reuse.

The ability to rework and recover components has become more importantto recover production yield loss, reduce waste, and provide costreduction in the fabrication of advanced technology high performanceelectronic products. Commonly used high thermal conductivity heatspreader materials include AlSiC, SiC (k=270), SiSiC (k=210), AIN, Al,Cu and other special types having low thermal expansion and high thermalconductivity, for example, CuW, diamond-SiC, surface metallized diamondsuch as with Ni, Cr, CrNiAu, diamond-like carbon etc., to confer otherdesirable properties as corrosion resistance and adhesion improvement.With the use of high cost diamond based heat spreaders which have thehighest thermal conductivity of all other common type of heat sinksemployed, having a rework option for the cured conductive films offers amajor benefit of recovery/reclamation and reuse, thus providing a costeffective way to obtain significant increase in heat dissipationcapability with the use of high thermal conductivity cooling element inconjunction with a thermal interface adhesive.

In view of the limitations in the use of conventional interfacematerials, there is a need for improved thermal interface materials(TIMs) with efficient heat dissipation from high power density devices.There is also a need for a practical method to rework the cureddeposits/residue of these materials from various componentsurfaces/interfaces the materials are adhered to.

BRIEF SUMMARY OF THE INVENTION

Briefly stated, in a first aspect the invention is a conductive adhesiverework method comprising steps of:

-   -   (a) providing a first cleaning solution for a thermal interface        cured coating/residue deposit on assembly components which        comprises tetramethylammonium fluoride (TMAF) or        tetrabutylammonium fluoride (TBAF), or a mixture thereof        dissolved in a first essentially water insoluble non-hydroxylic        aprotic solvent;    -   (b) submerging said assembly components carrying said cured        coating/residue of conductive adhesive thermal interface        material in said first cleaning solution heated at approximately        40 to 70° C. and allowing said assembly components to be        subjected to the cleaning action by said first cleaning solution        with stirring or agitation for a first predetermined period of        time between about 10 to about 90 minutes;    -   (c) removing said assembly components from said first cleaning        solution;    -   (d) transporting and submerging said assembly components in a        first solvent rinse bath which comprises a hydrophobic        non-hydroxylic solvent and subjecting said assembly components        to said first solvent rinse at approximately room temperature to        70° C. with agitation, for a second predetermined period of time        between approximately 5 to 15 minutes, to replace said first        cleaning solution on the assembly component surface with said        first solvent rinse;    -   (e) removing said assembly components from said first solvent        rinse bath;    -   (f) transporting and submersing said assembly components to a        second solvent rinse bath which comprises a hydrophilic        essentially water soluble solvent, and subjecting said assembly        components to a second solvent rinse at approximately room        temperature to 60° C. with agitation such as stirring or        immersion spray for approximately 5 to 10 minutes;    -   (g) removing said assembly components from said second solvent        rinse bath;    -   (h) transporting said assembly components to an aqueous rinse        bath and applying a water rinse, spray or immersion spray rinse,        at approximately room temperature to 50° C. for approximately 2        to 10 minutes;    -   (i) subjecting said assembly components to a third rinsing step        with IPA (isopropanol) to replace water on said component        assembly surface with IPA to accelerate drying; and    -   (j) drying said assembly components by blowing dry N₂ or air on        the surfaces and then heating said assembly components to        approximately 90° C. to 120° C. for approximately 30 minutes to        one hour to remove adsorbed moisture from said assembly        components.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description of a preferred embodiment of theinvention will be better understood when read in conjunction with theappended drawings. For the purpose of illustrating the invention, thereis shown in the drawings an embodiment which is presently preferred. Itshould be understood, however, that the invention is not limited to theprecise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a cross section of a single chip module with a metal plateattached to the back of the chip with conductive adhesive as thermalinterface.

FIG. 2 is a cross section of a single chip module with a protective capwhere the protective cap is attached to the back of the chip using asilicone polymer adhesive and a heat sink is attached to the protectivecap using conductive adhesive as thermal interface between the cap andthe heat sink.

FIG. 3 is a cross section of a dual chip module with a single heatspreader attached to the back of the two chips using conductive adhesiveas thermal interface material where one chip can have a thin bondlineand the second chip has a thicker bondline.

FIG. 4 is a cross section of a dual chip module with two separate heatspreaders bonded to each chip using conductive adhesive as interfacematerial.

FIG. 5 is a cross section of a multichip module with a metal plateattached to the back of the chips with a conductive adhesive as thermalinterface material.

FIG. 6 is a cross section of a multichip chip module with a heatspreader attached to the back of the chips using a conductive adhesiveas thermal interface and a protective cap attached using a siliconepolymer adhesive.

DETAILED DESCRIPTION OF THE INVENTION

The reworkable conductive adhesive compositions according to thisinvention are multi-component paste formulations containing a high levelof metal flake and/or powder filler dispersed in a polymer matrixderived from a liquid epoxy precursor preferably having a siloxanelinkage (—Si—O—Si—) and carrying an acyclic or alicyclic chain segment,standard solid or liquid anhydride or an amine curing additive, aconventional epoxy curing catalyst, a polymer additive which iscompletely miscible in the epoxy precursor, the later is added formatrix viscosity adjustment without the need to add a solvent, and theadditive is also found to provide improvement in the conductive adhesiveproperties.

These reworkable conductive adhesives, used as thermal interfacematerials, have shown superior properties as interface bonding materialsfor cooling plate/heat sink attachment to flip-chip in a single chipmodule (SCM), dual chip module (DCM) or an array of chips in amulti-chip module assembly (MCM). The cured adhesives can be readilyremoved from various heat sink surfaces and Si die backing by exposingthe disassembled components to a dilute solution of a quaternaryammonium fluoride in a non-polar aprotic solvent for a brief period oftime followed by rinsing with isopropyl alcohol (IPA) and drying.

The present invention also offers a major advantage of a reworkabilityoption, which is especially important in the recovery and reuse of themore expensive high thermal conductivity diamond based heat sinks andother module components. Most of the commercial epoxy based conductiveadhesives for thermal interface applications have high modulus and thushigher stress and present the problem of delamination under stressexposure, and the cured adhesives cannot be readily removed from thejoining surfaces.

The thermal interface materials of this invention are conductiveadhesives formulated by dispersing conductive metal filler particles ina solvent-free polymer matrix to obtain polymer-filler composite pasteof desired viscosity for the selected application. The polymer matrixused for these conductive adhesive compositions is a multi-componentsystem with viscosity suitable for dispersing a high level of metalflake and/or powder filler, the polymer matrix being derived from liquidepoxy precursors admixed with standard solid or liquid anhydride or anamine curing additive which are soluble in the liquid epoxide, astandard epoxy curing catalyst, and a low molecular polymeric additivesuch that it forms a completely miscible blend with the rest of thebinder matrix and having the desired viscosity to allow optimumconductive paste viscosity with high level of metal filler loadingwithout the need to add a solvent.

These compositions have the unique complement of properties desired foran improved TIM, particularly, low intrinsic stress, high thermalconductivity, no resin bleed during cure. The composite paste viscositycan be adjusted to provide thin bond line (<1 mil cured thickness) orthicker bond line (>1 mil), have low resistivity generally in the range10⁻⁵ Ω-cm, have high thermal conductivity (>3W/m° K.) for enhanced heatdissipation, stable interface thermal resistance, and good adhesion toall relevant surfaces, particularly Si chip backing and heat sinkmetals, and adhesion durability under reliability stress excursionsinvolving T/H (85° C./85%) and thermal cycling.

The polymer additive can be fully functionalized or have residualreactive sites which forms an interpenetrating network (IPN) upon curingof the conductive adhesive and affects the functional properties of theadhesive.

Preferred metal fillers for conductive paste compositions includePd-coated Ag, Au-coated Ag, Ag, Ag-coated Cu, spherical Ag powder,carbon fibers, particularly carbon microfibers, and combination thereof.The filler can also be a combination of electrically conductive metaland thermally conductive/electrically insulative inorganic filler suchas BN, AIN, where the filler can be in the form of flake, powder, hollowspheres, or fibers. The particle size of the filler can be primarilymonodisperse or polydisperse phase with varying particle sizedistribution, shape and morphology. The fillers that have averageparticle size less than 10 μm and have narrow particle size distributionthat assures high packing density are preferred. The Pd or Au arepreferably in an amount in the range of 5 to 20 wt % relative to Ag. Theparticle size of the metal filler varies in the range 2-30 μm.

The polymer matrix composition of this invention can allow dispersion ofthese fillers at a level ranging from 70-90% (wt %) without causingbrittleness, and enhanced thermal conductivity coatings are obtained athigh particle loading levels, preferably in the range 82-88% (wt. %).

The organic matrix binder system of the conductive adhesive compositionsas thermal interface materials for heat dissipation according to thisinvention is based on epoxy-low Tg compliant polymer additive which isprepared using commercially available precursor materials which are,liquid epoxide, conventional anhydride and amine curing agent, polymeradditive material of select chemistry, curing catalyst/cure acceleratorsystem.

Preferred liquid epoxide precursors are: bis(1,3-glycidoxypropyl)tetramethyl disiloxane; aliphatic diglycidyl ethers such asbis(1,4-butane diol) diglycidyl ether and mixture thereof; bis(1,5glycidoxy propyl)hexamethyl trisiloxane; 1,4-cyclohexane-dimethyldiglycidyl ether and related liquid cycloaliphatic diepoxides.

The curing agents used are preferably saturated aliphatic anhydrideswhich may be liquid or low melting solids which are miscible with theliquid epoxy precursor and forms a stable homogeneous mixture at roomtemperature or by heating up to 70-80° C. Representative candidates forsuitable anhydrides include: hexahydrophthalic anhydride (HHPA),hexahydro-4-methyl phthalic anhydride (MeHHPA), dodecynylsuccinicanhydride (DDSA); octenyl succinic anhydride; hexadecenyl succinicanhydride; cis-4-cyclohexane-1,2dicarboxylic anhydride orcis-1,2,3,6-tetrahydrophthalic anhydride (THPA);methyl-5-norbornene-2,3-dicarboxylic anhydride; maleic anhydride, andmixtures thereof.

The polymeric additive used in these epoxides can be an acrylate polymersuch as polyacrylate, poly(n-butylacrylate or n-butylmethacrylate) oflow molecular weight preferably having intrinsic viscosity <0.5;poly(n-fluorobutyl methacrylate), low molecular weight poly(methylmethacrylate) preferably having molecular weight <10,000, and mixturesthereof. Conductive adhesive compositions using this class of polymeradditive have been described previously in U.S. Pat. No. 6,548,175.

According to the present invention, it is discovered that alternate thelow Tg oligomeric additives having residual functional groups,specifically poly(acrylonitrile-co-butadiene-co-acrylic acid, dicarboxyterminated glycidyl methacrylate diester (ABA-glycidyl methacrylatediester) form a highly compatible blend with the epoxy precursor andprovide improved adhesive properties of the conductive adhesives derivedtherefrom when used with anhydride curing system, while anotherfunctionalized oligomeric additive amine terminatedpoly(acrylonitrile-co-butadiene) can be used with advantage in aminecuring conductive adhesive compositions. The low Tg polymer/oligomericadditive is in the range 5% (wt %) to about 30% (wt %), preferably inthe range 10 wt % to 20 wt % by weight of the epoxy-polymer additivemixture, the rest being the epoxy precursor.

It is also found that in the anhydride curing formulations, using amixture of anhydrides, for example, MeHHPA and HHPA; DDSA and HHPA; orMeHHPA and MA results in superior conductive adhesive properties than isgenerally observed with using a single type of anhydride curing agent.The preferred mole ratio of the anhydride curing agent to the epoxyequivalent is in the range 1:1 to 1:2, respectively. When using amixture of two anhydrides, the relative ratio of the anhydrides can bein the range 1:2 to 2:1.

Preferred Au-coated Ag filler have about 10% Au and 90% Ag, which has50% particle size <2.4 μm, 95% is <5.9 μm, and 10% is <0.77 μm.Depending on the particle size, distribution, and morphology, thepolymer matrix composition of this invention can allow dispersion offiller at a level ranging from 70-90% (wt %) to obtain conductive pasteviscosity suitable for manual dispense, screen or stencil printing, orwith an auto-dispense tooling. Typical viscosity of freshly formulatedconductive adhesives for thermal interface application can be in therange 20,000 to 60,000 Pa/S.

In a representative example of preparing the conductive adhesive pasteformulation, 5% to about 30% (wt %) of the polymeric/oligomeric additiveis added to the liquid epoxide and the mixture allowed to stir at roomtemperature or at elevated temperature till it forms a homogeneousblend. The anhydride curing additive is then added and the mixture isstirred at about 50-70° C. for 30 minutes to completely dissolve theanhydride.

In an alternate procedure, epoxy/anhydride mixture is formed first andthen the polymeric/oligomeric additive is blended in with mechanicalmixing till a clear mixture is formed without requiring solventaddition. A catalyst/accelerator system used in conjunction withanhydride curing epoxy formulations is added which commonly includes atertiary amine, typically, 2,4,6-tris(dimethylaminomethyl)phenol,benzyldimethyl amine (BDMA), 2,6-diaminopyridine along with a protonsource, typically nonylphenol, ethylene glycol, resorcinol, and relatedmaterials. The amine accelerator can be in the range 0.02 to 0.5% (wt %)of the epoxy-polymer additive-anhydride mixture. The proton source usedin conjunction with the amine catalyst can be in the range 0.1 to 1.0%of the total organic binder mixture.

All the organic components are thoroughly mixed together and thecatalyzed system can either be used immediately for dispersing the metalfiller to form conductive paste or it can be stored at −20 or −40° C.for later use. Conductive metal filler is then dispersed in thecatalyzed organic matrix by adding in portions and constant mixing witha rotary mixer, the amount of metal filler added varies between 70-85 wt% depending on the filler type, to obtain paste viscosity suitable forforming a defect-free thin bond line coatings by manual dispense,syringe dispensing or screen printing on silicon chip and metal heatsink surfaces. After a homogeneous paste consistency is obtained for adesired application, the paste is deairated to remove any trapped airand stored at a minimum of 40° C. when not in use.

Curing and characterization of representative conductive adhesivesdescribed here for thermal interface application was conducted byforming thin coatings on glass slides, ceramic substrates, siliconwafers, and on various metal heat sinks, and subjecting them to thermaltreatment at 90-100° C. for 30 minutes followed by 160-175° C. for 60-90minutes, preferably in a N₂ purged oven. Curing behavior of theadhesives was evaluated by differential scanning calorimetry (DSC) whichshowed exothermic transition with peak temperature ranging from 150° C.to 175° C. for the anhydride cure adhesives, the heat generally observedwas in the range 35 to 60 J/g depending on the adhesive chemistry.Thermal stability was tested by carrying out thermogravimetric analysis(TGA) from room temperature to 250° C. at 10°/min ramp rate and also byisothermal TGA at 180° C. for extended period of time.

The TGA data for specific materials are provided in table 1. Forresistivity measurements, the adhesives pastes were dispensed onto glassslides to form strips having about 4 cm length, 1 cm width and 0.8 to1.2 mil wet thickness which on curing gives <1 mil coating thickness.

A typical test method for the use of these adhesives as thermalinterface materials in heat sink attachment involved dispensing theconductive paste onto the back side of silicon chip, size 18 mm² or 15.2mm by 16.2 mm x,y size, mounted on a ceramic chip carrier (aluminaceramic, 42.5 by 42.5 mm, 2 mm thick substrate) by C4 Pb/Sn solder (97%Pb/3% Sn), or on a heat sink surface, spreading the paste to form a thinuniform layer, securing the coated component in a fixture withindividual slots and placing a SiSiC heat sink/heat spreader on top inalignment with the chip backing and secure the assembly by clamp downwithout disrupting the adhesive contact interface. The assembly is thencured in a N₂ purged oven 100-110° C. for 40 minutes followed by160-170° C. for 90 minutes, allowed to cool down to at least 80° C.before removing the clamps. Shear strength is measured according tostandard method of tensile pull using Instron. The relevant time zeromeasurement data are shown in table 1.

TABLE 1 Anhydride TGA Rt- TGA Shear Curing Polymer Conductive 250° % wtIsothermal/ strength, Example Agent^(a) additive^(b) Metal Filler losshr · 180° C. Psi 1 MeHHPA + HHPA n-BuMA Au-coated Ag 0.59 0.21 2050polymer flake/powder 2 MeHHPA + HHPA + NMA N0BuMA Au-coated Ag 1.0 0.251250 polymer flake/powder 3 DDSA + MA n-BuMA Au-coated Ag 0.5 0.15 1400polymer flake/powder 4 DDSA n-BuMA Pb-coated Ag 0.55 0.2 1300 polymerflake/powder 5 HHPA ABGMA Au-coated Ag 0.5 0.18 1950 oligomerflake/powder 6 HHPA ABGMA Ag flake 0.9 0.34 1360 oligomer 7 DDSA n-BuMAAu-coated Ag 0.55 0.16 1350 polymer flake/powder ^(a)HHPA =Hexahydrophthalic anhydride; DDSA = Dodecenyl succinic anhydride; MeHHPA= 4-Methyl Hexahydrophthalic anhydride. ^(b)Epoxide =1,3-bis(glycidoxypropyl) tetramethyldisiloxane c. nBuMA poly. =Poly(n-butyl methacrylate), intrinsic viscosity about 0.5, PMMA = Poly(methyl methacrylate), avg Mw 15,000 ABGMA polymer =Poly(acrylonitrile-co-butadiene-co-acrylic acid, dicarboxy terminatedglycidyl methacrylate diester (ABA-glycidyl methacrylate diester).

The representative conductive adhesives described for use as improvedthermal interface materials with heat dissipation elements generallyprovide low modulus cured adhesives having room temperature modulus of8000-40,000. They have a high conductive filler loading without causingbrittleness and provide high thermal conductivity and higher electricalconductivity as compared to the high modulus commercial Ag-filledepoxies.

The paste compositions chemistry and viscosity can be adjusted to obtainthin bond line without voids, defects or resin bleed. There is nosignificant change in contact resistance or interfacial voids/defectsobserved on thermal cycling at 0-100° C., 500-1000 cycles. The epoxymatrix has been modified by incorporation of a low Tg polymeric and/oroligomeric system to confer special properties to the cured conductiveepoxy polymer, low stress/flexibility/compliance, minimal change inshear strength upon exposure to reliability stress environment, and longterm heat dissipation performance of cooling elements.

The conductive adhesives offer potential for enhanced thermalperformance by incorporation of a phase change material (PCM) asadditive in the matrix or by lamination of the preform. A practicalmethod is provided for reworking/removing these cured adhesives fromheat sinks, silicon device chip, and in plastic package assemblyproducts which offers a major benefit of component reclaim, defectrepair, and cost reduction by yield loss recovery through rework.

As a representative example, liquid siloxane epoxide precursor,1,3-bis(glycidoxypropyl)tetramethyl disiloxane, and the anhydride curingagent which may be liquid or low melting solid, are mixed together inthe preferred mole ratio of the anhydride curing agent to the epoxyequivalent in the range 1:1 to 1:2, respectively. When using a mixtureof two anhydrides, the relative ratio of the anhydrides can be in therange 1:2 to 2:1. The mixture is stirred at about 50-70° C. for 30minutes to completely dissolve the anhydride in the siloxane epoxide.The polymer additive, poly(n-butyl methacrylate) or poly(methylmethacrylate) or ABA-glycidyl methacrylate diester is then added to theepoxide-anhydride solution in an amount such that it is present in anamount 5 to 20 wt % based on the total organic binder which includessiloxane epoxide, anhydride, polymer additive, and the cure catalyst,and the mixture is stirred for several hours at 50-70° C. until thepolymer essentially dissolves and a homogeneous clear viscous solutionis formed.

This mixture is then allowed to cool to room temperature and the curecatalyst system is added which are commonly used in epoxy-anhydride cureand which includes a tertiary amine, typically,2,4,6-tris(dimethylaminomethyl)phenol, benzyldimethyl amine (BDMA), anda proton source as nonylphenol, ethylene glycol and related materials.All the organic components are thoroughly mixed together and thecatalyzed system can either be used immediately for dispersing the metalfiller to form conductive paste or it can be stored at −20 or −40° C.for later use. Conductive metal filler is then added in portions to theabove catalyzed organic carrier composition with constant mixing with arotary mixer, the amount of metal filler added varies between 70-85 wt %depending on the filler type, to obtain paste viscosity such that it issuitable for screen printing and for syringe dispensing. The stirringrate is maintained for steady mixing without causing excessiveentrapment of air bubbles. After a homogeneous paste consistency isobtained for a desired application, the paste is de-aireated to removeany trapped air bubbles and then stored at a minimum of −40° C. when notin use.

To thermally cure the metal filled adhesives described above, the pasteis applied as a thin layer onto a substrate, for example, ceramic,silicon, Au surface, and Pb/Sn solder, and cured in an oven preheated at100° C. under N₂ and the adhesive paste is cured at 100-110° C. forabout 30 minutes followed by 160-170° C. for 60 minutes, the heat wasthen turned off and the cured adhesive samples were allowed to cool toroom temperature in the oven. To obtain free standing films formeasurement of elastic modulus, the conductive formulations are cured ina mold made of a material having low energy surface, the mold havingknown depth, width and length is typically made of Teflon, by dispensingthe adhesive paste in to the mold and curing at 100-110° C. for about 30minutes followed by 160-170° C. for about 60-90 minutes. After coolingto room temperature, the cured samples are removed from the mold andcharacterized for elastic modulus according to the standard methodsusing instron tensile test.

The following are examples of the representative adhesive compositionsshown in Table 1:

EXAMPLE 1

A soluble mixture of 7.8 g of a 1,3-bis(glycidoxypropyl)tetramethyldisiloxane and 1.3 g poly(n-butylmethacrylate) was prepared by heatingwith stirring at 70° C., then 2.2 g MeHHPA and 2.5 g HHPA was added andagain allowed to stir until a clear mixture was formed. To this mixturewas added 0.14 g nonylphenol+0.05 g ethylene glycol, and 0.03 g of thetertiary amine 2,4,6-tris(dimethylamino-methyl)phenol (DMP-30) andthoroughly mixed to form a clear homogeneous blend. About 65.5 g ofAu-coated Ag filler were blended in this catalyzed mixture to formscreenable conductive adhesive paste having about 82.5 wt % fillerloading. The material was characterized for relevant properties andtested as a thermal interface material in conjunction with a SiSiC heatspreader. The data is summarized in table 1.

EXAMPLE 2

This example was a repeat of Example 1 except that in addition to MeHHPAand HHPA, about 0.2 g of methylnadic anhydride (NMA) was added to thecomposition described in Example 1. The filler type and % loading werekept the same. Relevant data on the characterization of the curedadhesive derived from this conductive paste are shown in table 1.

EXAMPLE 3

A soluble mixture of 17.5 g of a 1,3-bis(glycidoxypropyl)tetramethyldisiloxane, and 3.2 g of poly(n-butylmethacrylate) prepared accordingthe method of Example 1. To 7.2 g of this mixture was added 5.5 g DDSAand 0.5 g MA, stirred to dissolve contents, and the catalyst system, 0.1g nonylphenol and 0.15 g of 2,4,6-tris(dimethylaminomethyl)phenol(DMP-30) were added and thoroughly mixed to form a clear homogeneousblend. To obtain the conductive paste, about 65 g of Ag/Au filler (90%Ag/10% Au wt % ratio), described in Example 1, was blended in thiscatalyzed mixture. The material was characterized for relevantproperties and tested as a thermal interface material in conjunctionwith a SiSiC heat spreader. The data is summarized in table 1.

EXAMPLE 4

Dodecenylsuccinic anhydride (DDSA), 2.6 g was added to a solution ofabout 2.8 g of 1,3-bis(glycidoxy-propyl)tetramethyldisiloxane and 0.6 gof poly(n-butyl methacrylate) prepared by dissolving the polymer in theliquid siloxane epoxide, and heated at 50° C. with stirring until aclear viscous solution was formed. The solution was allowed to cool toroom temperature and then 0.035 g of nonylphenol, 0.025 g of ethyleneglycol and 0.06 g 2,4,6-tris(dimethylaminomethyl) phenol (DMP-30) wereadded and well mixed to form a clear homogeneous solution. To about 3.8g of the final catalyzed mixture was blended in 14.6 g Ag/Pd metalfiller according to the method described in the above examples to form aconductive adhesive paste having about 79.4% (wt %) filler loading.

EXAMPLE 5

A mixture of 3.2 g of a 1,3-bis(glycidoxypropyl)tetramethyl disiloxaneand 0.7 g of poly(ABA-glycidyl methacrylate oligomer) was allowed tostir until a clear blend was formed and then 2.2 g hexahydrophthalicanhydride solid (HHPA) was added and the mixture allowed to stir forabout 30 minutes until a clear viscous solution formed. To this mixturewas added 0.05 g nonylphenol, 0.02 g ethylene glycol and 0.045 g DMP-30and the contents thoroughly mixed till it formed a clear homogeneoussolution. About 27 g of Ag/Au filler (90% Ag/10% surface coated Au, wt %ratio) was blended in this catalyzed mixture according to general methoddescribed above to form a conductive adhesive paste having about 81.6wt. % conductive particles. The paste was characterized for propertiesrelevant to application as a thermal interface adhesive and functionallytested in heat sink attachment for shear strength and its durabilityunder stress. The data is collected in table 1.

EXAMPLE 6

A mixture of 3.57 g of a 1,3-bis(glycidoxypropyl)tetramethyl disiloxaneand 0.73 g of poly(ABA-glycidyl methacrylate oligomer) was allowed tostir until a clear blend was formed and then 2.1 g 4-methylhexahydrophthalic anhydride was added and the mixture allowed to stirfor about 30 minutes when a clear viscous solution formed. To about 3.2g of the mixture was added 0.04 g nonylphenol, 0.04 g ethylene glycoland 0.07 g DMP-30 and the contents thoroughly mixed till it formed aclear homogeneous solution. About 21 g of Ag flake SF9AL was blendedinto the organic mixture to form a conductive paste of desiredviscosity.

EXAMPLE 7

Example 4 was repeated exactly as described in terms of the catalyzedbinder system with the difference that the filler used was Ag coated Auwith about 81.5% (wt %) filler loading. Table 1 shows the relevantproperties of the conductive adhesive.

Rework is another aspect of the present invention. Micro-electronicsfabrication processes often require disassembly of assembled components.Typical reasons include carrying out diagnostic tests, to replace orrepair the semiconductor device, or to recover electrically goodsubstrates from test vehicles or early user hardware used to assessproduct performance and reliability prior to actual product release.FIG. 1 shows a typical single chip module 10 having a ceramic chipcarrier or substrate 11 with a single chip 12 attached through solderjoints 13, which are encapsulated with an underfill polymer 14. For heatdissipation from the functioning device, a thermal interface materiallayer 15 (TIM) of the present invention is dispensed on the back sidesurface of the chip 12 and a metal heat sink or a metal plate 16 isbonded to the die with the conductive adhesive 15 as the thermalinterface material for heat sink attached assembly.

FIG. 2 shows a conventional single chip module assembly as in FIG. 1 butadditionally shows a protective cap 16 attached to the substrate with anelastomeric silicone polymer adhesive and a heat sink or heat slug 20attached to the protective cap 16 with a conductive adhesive as thethermal interface material 19. FIGS. 3 and 4 show a dual chip module(DCM) represented in two versions as 21 a and 21 b. A ceramic chipcarrier or substrate 22 with two chips 23 a and 23 b attached throughsolder joints 24 is shown with the solder joints being encapsulated withan epoxy encapsulant 25. A single protective metal cap 26 is bonded tothe substrate with an elastomeric silicone polymer adhesive.

The silicon chips 23 a and 23 b mounted on the chip carrier in flip-chipconfiguration contact with the metal cap through bonding with theconductive epoxy adhesive as the thermal interface material 27 a and 27b. The two adhesives can both be the conductive adhesives of the presentinvention, or one can be a conventional material such as a conductivesilicone polymer and the second can be one of the TIM disclosedaccording to the present invention.

The chip size can be different, for example, one chip 15.4 by 17.9 mmand the second chip smaller, 13.2 by 15.9 mm, the substrate size 42.5 by42.5 mm. FIG. 4 shows a ceramic chip carrier or substrate 22 with twochips 23 a and 23 b attached through solder joints 24 with the solderjoints being encapsulated with an epoxy encapsulant 25. The chip 23 a isa lower power chip to which a metal plates/heat sink is directlyattached (DLA) with a conventional silicone adhesive 27 a, and the chip23 b is a high power chip to which a heat sink is attached using thehigh thermal conductivity interface polymeric adhesives 27 b.

FIG. 5 illustrates a typical multi-chip module (MCM) 31 where a ceramicchip carrier substrate 32 is connected to a plurality of chips 33through solder joints 34 with the epoxy encapsulant 35 covering thesolder joints. A thermal paste or a conductive epoxy thermal interfacecompound 36 of the present invention is shown as the adhesive interfacematerial on the back side surface of silicon chips 33 for heat sinkattachment to provide the necessary cooling or heat dissipation from thechip.

FIG. 6 shows a protective cap 37 attached to chip carrier 32 with apolymer adhesive 38. FIG. 6 is an illustration of the MCM of FIG. 5shown generally as 39 having a metal heat sink 41 attached to theprotective cap 37 with a thermally and electrically thermal interfaceadhesive 40.

A representative heat spreader/module assembly process using thesereworkable conductive adhesives as thermal interface involves the stepsof:

applying a thin layer of conductive paste by dispensing or screenprinting onto the back of device chips mounted on the chip carrier orthe conductive adhesive can be dispensed on the heat spreader, or it canbe dispensed on both the heat spreader and the device chip back side;

securing the adhesive coated chip/module assembly in a clamping fixturewith slots matching the substrate size;

aligning the heat spreader with back of the chips and placing in contactwith the adhesive coated chip surface and clamping the assembly toprevent component movement and provide pressure during subsequentcuring;

placing the assembly in a preheated oven at 90-100° C., holding at thistemperature for 45 minutes, then ramping the temperature to 160-170° C.and holding at this temperature for 60-90 minutes for complete cure ofthe interface adhesive.

Removal processes for various assembly materials must be selective for aparticular material set and cause no detriment to the substrateintegrity and electrical performance. It is also required that theremoval method be environmentally and chemically suitable for use in amanufacturing environment.

Unlike the commercial Ag-filled mostly non-reworkable thermal interfaceepoxy adhesives, the cured conductive adhesives disclosed according tothe present invention for improved thermal interface materials can beeasily reworked by subjecting the component carrying the adhesiveresidue to a dilute solution of a quaternary ammonium fluoride,preferably tetrabutylammonium fluoride (TBAF) 1-2 wt % in a non-polaraprotic solvent, preferably in the presence of a surface active agentcomprising a non-ionic surfactant or a combination of an amphoteric anda non-ionic surfactant, under very mild conditions requiring less than10-20 minute immersion time at 40-50° C., followed by rinsing with a lowboiling hydroxylic solvent, typically IPA followed by dry and bake.

Rework methods for Sylgard and related cured silicone polymers usingTBAF based compositions has been detailed in the U.S. Pat. No.6,652,665. According to the present invention, it is discovered that thecured conductive adhesives described here can also be reworked with theTBAF derived solutions found effective in removing Sylgard residues fromvarious component surfaces.

Therefore, by providing the adhesives reworkability option, the presentinvention offers an opportunity to rework and recover high costelectronic components such as high performance heat sinks, allow defectrepair to recover yield loss and reduce product cost, and reclaim testvehicles which are currently discarded for lack of a suitable rework Themethod is based on non-alkaline or mildly alkaline solution chemistryand has no environmental and health hazard concerns, no chemical safetyor flammability issues for use in manufacturing environment. It iscompatible with all type sensitive metallurgical interconnectionsincluding conventional C4s and Pb-free joints.

In this additional embodiment, the present invention provides animproved method of removing cured conductive polymer adhesives which aredisclosed here as thermal interface materials from electronic componentsto offer an option for rework, to repair defects, and for reclamation orrecovery of usable parts of the assembly products, particularly the highcost heat sinks and other module components. Yet another attribute ofthe rework chemistry found effective in stripping conductive adhesivelayer as thermal interface is its compatibility with ceramic modulescarrying silicon device chips as well as with plastic packages.

The rework method comprises the steps of:

(a) providing a first stripping solution for the thermal interfacecoating deposit on a heat sink which comprises tetramethylammoniumfluoride (TMAF) or tetrabutylammonium fluoride (TBAF), or a mixturethereof dissolved in a first essentially water insoluble non-hydroxylicaprotic solvent, for example, propylene glycol methyl ether acetate(PGMEA)

(b) submerging the electronic components carrying the curedcoating/residue of conductive adhesive thermal interface material in thefirst cleaning solution heated at 40 to 70° C., preferably 45 to 60° C.and allowing the components to be subjected to the cleaning action bythe solution with stirring or agitation for a first predetermined periodof time between about 10 to about 90 minutes, depending on the extent ofpolymer residue and the component surface topography;

(c) removing the assembly components from the first cleaning solution;

(d) transporting and submerging the composition in the first solventrinse bath which comprises a hydrophobic non-hydroxylic solvent,preferably the same solvent as used for the first cleaning solution, andsubjecting the components to the solvent rinse, for example, immersionrinse at room temperature to 70° C. with agitation, for a secondpredetermined period of time between about 5 to about 15 minutes, toreplace the cleaning solution on the component surface with the solvent;

(e) removing the components from the first solvent rinse bath;

(f) transporting and submersing the components to the second solventrinse bath which comprises a hydrophilic essentially water solublesolvent, and subjecting the components or parts to the second solventrinse at room temperature to about 60° C. with agitation such asstirring or immersion spray for about 5 to 10 minutes;

(g) removing the components from the second solvent rinse bath;

(h) transporting the components to an aqueous rinse bath and applying awater rinse, preferably deionized water rinse, for example, spray orimmersion spray rinse, at room temperature to about 50° C. for 2 to 10minutes;

(i) subjecting the components to another brief rinsing step with IPA(isopropanol) to replace water on the component surface with IPA toaccelerate drying;

(j) drying the components by blowing dry N₂ or air on the surfaces andthen heating the assembly components at about 90° C. to about 120° C.for 30 minutes to about one hour, preferably under vacuum to removeadsorbed moisture from the components.

In an alternative solvent rinse process, the assembly components orparts after the first solvent rinse in non-hydroxylic aprotic solventsuch as PMA, are transported to a second solvent bath also containing ahydrophobic non-hydroxylic solvent, preferably the same solvent as usedfor the first cleaning solution and the first rinse solvent such as PMA,and subjecting the parts to the second solvent rinse similar to thefirst solvent rinse. After the second solvent rinse, the assemblycomponents are transported to a bath containing IPA where the parts aresubjected to a spray rinse or immersion rinse with IPA to replace thePMA solvent with IPA, and then dried by blowing dry N₂ or air on thesurface followed by heating the component parts at about 90° C. to about120° C. for 30 minutes to one hour, preferably under vacuum.

The preferred quaternary ammonium fluoride (QAF) compound in the firstcleaning solution is tetramethylammonium fluoride (TMAF),tetrabutylammonium fluoride (TBAF), tetraoctylammonium fluoride or amixture thereof, which is present at a concentration of about 0.2 to 5weight %, preferably 0.5 to 1% based on the formula (C₄H₉)₄N⁺F⁻, or 0.6to 1.5% (weight %) as the trihydrate (TBAF.3H₂O) in hydrophobic aproticsolvent, preferably propylene glycol methyl ether acetate (PMA). Othersolvents as replacement of PMA include tetrahydrofuran (THF),acetonitrile, chlorobenzene etc.

The first solvent rinse bath comprises a non-hydroxylic aprotic solventwhich is preferably the same solvent as in the first cleaning solutionsolvent in the category of propylene glycol alkyl ether alkoate selectedfrom the group consisting of propylene glycol methyl ether acetate(PMA), propylene glycol ethyl ether acetate (PGEEA, bp. 158° C.),propylene glycol methyl ether propionate (methotate), di(proyleneglycol) methyl ether acetate (DPMA, bp. 200° C.), ethoxy ethylpropionate (EEP).

The second rinse solvent is a hydrophilic essentially water solublesolvent represented by propylene glycol alkyl ethers selected from thegroup consisting of di(propylene glycol) methyl ether (DPM, fp 75° C.),tri(propylene glycol) monomethyl ether (TPM, fp 96° C.), tri(propyleneglycol)n propyl ether, or a mixture thereof, used at a temperature fromabout room temperature to about 60° C.

In the alternative solvent rinse process, the parts after the firstsolvent rinse in PMA or related non-hydroxylic aprotic solvent are againsubjected to the same solvent rinse, preferably PMA in a second solventbath followed by spray or immersion rinse in IPA, and dried by blowingdry N₂ or air on the surfaces followed by heating the component parts atabout 90° C. to about 120° C. for 30 minutes to one hour, preferablyunder vacuum.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is to be understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A conductive adhesive rework method comprising the steps of: (a)providing a first cleaning solution for a thermal interface curedcoating/residue deposit on assembly components which comprisestetramethylammonium fluoride (TMAF) or tetrabutylammonium fluoride(TBAF), or a mixture thereof dissolved in a first essentially waterinsoluble non-hydroxylic aprotic solvent; (b) submerging said assemblycomponents carrying said cured coating/residue of conductive adhesivethermal interface material in said first cleaning solution heated atapproximately 40 to 70° C. and allowing said assembly components to besubjected to the cleaning action by said first cleaning solution withstirring or agitation for a first predetermined period of time betweenabout 10 to about 90 minutes; (c) removing said assembly components fromsaid first cleaning solution; (d) transporting and submerging saidassembly components in a first solvent rinse bath which comprises ahydrophobic non-hydroxylic solvent and subjecting said assemblycomponents to said first solvent rinse at approximately room temperatureto 70° C. with agitation, for a second predetermined period of timebetween approximately 5 to 15 minutes, to replace said first cleaningsolution on the assembly component surface with said first solventrinse; (e) removing said assembly components from said first solventrinse bath; (f) transporting and submersing said assembly components toa second solvent rinse bath which comprises a hydrophilic essentiallywater soluble solvent, and subjecting said assembly components to asecond solvent rinse at approximately room temperature to 60° C. withagitation such as stirring or immersion spray for approximately 5 to 10minutes; (g) removing said assembly components from said second solventrinse bath; (h) transporting said assembly components to an aqueousrinse bath and applying a water rinse, spray or immersion spray rinse,at approximately room temperature to 50° C. for approximately 2 to 10minutes; (i) subjecting said assembly components to a third rinsing stepwith IPA (isopropanol) to replace water on said component assemblysurface with IPA to accelerate drying; and (j) drying said assemblycomponents by blowing dry N₂ or air on the surfaces and then heatingsaid assembly components to approximately 90° C. to 120° C. forapproximately 30 minutes to one hour to remove adsorbed moisture fromsaid assembly components; wherein the thermal interface curedcoating/residue deposit is a cured product of a reworkable conductiveadhesive composition comprising conductive filler particles dispersed ina solvent-free epoxy polymer matrix; wherein the solvent-free epoxymatrix comprises a liquid epoxy precursor free of siloxane linkages, asolid or liquid anhydride curing additive or a solid or liquid aminecuring additive, a curing catalyst, and a polymer additive completelymiscible in said epoxy precursor.
 2. The method of claim 1 wherein saidfirst cleaning solution additionally contains a surface active agent inthe amount of approximately 0.1 to 1.0% (wt/vol %) in the cleaningsolution.
 3. The method of claim 2 wherein the surface active agent is anon-ionic surfactant or an amphoteric surfactant, or a combinationthereof.
 4. The method of claim 1 wherein said first essentially waterinsoluble non-hydroxylic aprotic solvent is propylene glycol methylether acetate (PGMEA).
 5. The method of claim 1 wherein said water rinseis a deionized water rinse.
 6. The method of claim 1 wherein said stepof drying said assembly components by blowing dry N₂ or air on thesurfaces and then heating said assembly components to approximately 90°C. to 120° C. for approximately 30 minutes to one hour is performedunder vacuum.
 7. The method of claim 1 wherein said first solvent rinsebath is selected from the group consisting of: propylene glycol methylether acetate (PMA), propylene glycol ethyl ether acetate (PGEEA, bp.158° C.), propylene glycol methyl ether propionate (methotate),di(propylene glycol) methyl ether acetate (DPMA, bp. 200° C.), andethoxy ethyl propionate (EEP).
 8. The method of claim 1 wherein saidsecond solvent rinse bath is selected from the group consisting of:di(propylene glycol) methyl ether (DPM, fp 75° C.), tri(propyleneglycol) monomethyl ether (TPM, fp 96° C.), tri(propylene glycol)n-propyl ether, and mixtures thereof.